In nature, biogenic crystals are often formed via amorphous precursors rather than classical nucleation and growth. Crystals grown in the laboratory by classical methods of nucleation and growth have facets dictated by the atomic structure and minimization of the surface free energy of the different orientation, wherein the facets are the low energy crystallographic planes (Vlachos et al., 1993). On the other hand, a remarkable outcome of crystal growth via an amorphous precursor is the ability of the organism to obtain unfaceted sculptured single crystals with rounded shapes including such crystals that are highly porous. Another outstanding advantage of growing crystals via this non-classical route is the fact that those crystals can be molded into any desired shape (Weiner et al., 2005, Weiner and Addadi, 2011, Fratzl et al., 2010, Sommerdijk and Cölfen, 2010).
Attempts have been made to mimic the growth of biogenic crystals with intricate shapes and morphologies (Cölfen, 2008). Thus, e g., single crystals of calcite were grown through the amorphous calcium carbonate precursor phase on micropatterned templates induced by a self-assembled monolayer on the template surface (Aizenberg et al., 2003), and single crystals of, e.g., calcium carbonate, lead(II)sulfate, and strontium sulfate were grown, using a sponge-like polymer membrane as a template, while being constrained into intricate shapes using that template (Wucher et al., 2007; Yue et al., 2006; Meldrum et al., 2007, Park and Meldrum, 2002). These processes were carried out using ceramic materials found in biogenic material such as CaCO3, but no such attempts were made with widely used functional technological materials such as metals. Fashioning single crystals having curved intricate shapes from functional materials such as noble metals would have high research and technological potential, e.g., in photonics (micro-lenses and micro-mirrors) (Audran et al., 2010, Lee et al., 2012). Such crystals having highly porous morphology may be used in drug delivery or high adsorption sensors.
With such functional materials as the starting point, however, obtaining the curved morphologies achieved in biogenic crystals requires additional fabrication steps such as sculpturing, drilling, and polishing the single crystals, or using standard nano- and micro-fabrication techniques (Audran et al., 2010; Corso et al., 2009; Lee et al., 2008). Polishing and microfabrication have been carried out on curved crystals obtained with gold, but the curvature was on a scale of millimeters and not microns (Corso et al., 2009). On the other hand, to the best of our knowledge, these procedures nave not been applied on micron-sized single crystals.